This invention relates to the field of optical spectrometers, and more particularly to holographic Fourier transform spectrometers (HFTS), and more particularly to an aberration-limited HFTS having improved performance due to the digital aberration correction.
The idea of using optical interference for retrieval of spectral information has a long history, dating back to the 19th century and works by H. Fizeau, L. Foucault and later A. A. Michelson. Rapid development of interferometric spectroscopy in the second half of the 20th century started with groundbreaking works by Fellgett and Jacquinot, who were first to document the multiplex and throughput advantages of the Fourier spectroscopic technique. By the 1960's step scan interferometers dominated the far infrared spectral region, but due to the slowness of the computers available at that time an incentive existed to eliminate the computing step of the conversion of the interferogram into its corresponding spectrum.
The first successful application of optical processing for restoration of spectral information from a hologram was demonstrated in 1965, but this application involved the scanning of one arm of the interferometer, while moving the recording medium at the same speed. The first demonstration of a completely static Fourier transform spectrometer (FTS) was published at about the same time. In this experiment a two beam interference pattern was produced in a birefringent crystalline quartz prism and reimaged on a photographic plate. After digitization, the interferogram was converted into a spectrum using a computer. A few other examples of a holographic FTS (HFTS) with optical transformation of the interferogram appeared later, using a tilted mirror in a Michelson-Twyman-Green interferometer, shearing Sagnac interferometer, Lloyd's mirror, and a modified Mach-Zehnder interferometer. But the rapid progress of digital computing power, the invention of a fast Fourier transform algorithm, and the broad use of photoelectric radiation detectors instead of photographic plates has made the optical Fourier transformation of interferograms unnecessary.
With scanning FTS firmly established in the far infrared and infrared, and grating spectrometers dominating the UV/visible region, where due to predominantly photon noise the Fellgett multiplex advantage does not exist, the development of HFTS devices appeared to reach steady state. However, a new thrust in this area started when the static interference pattern in the output of the interferometer was sampled with a multichannel photodiode detector, connected to a microcomputer. The resulting capability to provide imaging information in one direction with the spectral information in the other, combined with a compact and rugged design with no moving parts, made HFTS very attractive for remote sensing (often called digital array scanned interferometers or DASI, U.S. Pat. No. 4,976,542). Holographic FTS also were successfully tested in astronomy, single event rapid spectroscopy, Raman spectroscopy, and toxic gases monitoring.